The present invention concerns a method for the production of uranium from rock phosphate.
Native rock phosphate contains as a rule uranium in an amount of from 100-200 ppm and various methods have been proposed in literature for the recovery thereof.
Phosphoric acid is produced from rock phosphate by the so-called wet process which comprises decomposition of the rock phosphate with aqueous sulfuric acid. The wet process has two modifications. By one of these, known as the dihydrate method, the calcium values of the native phosphate rock are precipitated in the form of calcium sulfate dihydrate CaSO.sub.4.2H.sub.2 O, which produces a phosphoric acid of a concentration expressed in terms of P.sub.2 O.sub.5 contents of up to about 27-32% by weight. By the other modification, known as the hemi-hydrate method, the calcium values of the native phosphate rock are precipitated in the form of calcium sulfate hemi-hydrate-CaSO.sub.4.1/2H.sub.2 O and this yields a phosphoric acid of a concentration of 40-45% by weight of P.sub.2 O.sub.5. In either case the uranium values of the native rock are present in the crude wet process phosphoric acid and it is the object of the present invention to provide an efficient liquid-solid ion exchange method for the recovery of uranium from crude wet process phosphoric acid.
Current processes for the recovery of uranium from the wet process phosphoric acid (hereinafter for short WPA) are based on liquid-liquid extraction techniques which apply selective organic solvents such as, for example, octyl-pyrophosphoric acid known as OPPA, a mixture of di-(2-ethylhexyl)phosphoric acid and trioctylphosphine oxide in kerosene diluent known as DEHPH-TOPO or octyl phenyl phosphoric acid known as OPAP. By these processes uranium is recovered from crude 28-32% P.sub.2 O.sub.5 WPA. Impurities in the WPA, mainly organic matter and finely dispersed solids, cause difficulties in the operation of these liquid-liquid extraction processes such as interfacial crud formation, hindered phase separation, solvent losses, and barren acid contamination with traces of solvents. Because of these problems, appropriate cleaning of the WPA prior and after the extraction of uranium therefrom is essential. However, this in turn renders the entire extraction process more complicated and expensive. Moreover, it has been found that by known extraction operations it is practically impossible to extract uranium efficiently from 40-45% P.sub.2 O.sub.5 WPA obtained by the so-called hemihydrate wet process.
In view of the difficulties encountered in liquid-liquid extraction of uranium values from WPA, it has already been proposed to effect such recovery by means of liquid-solid ion exchange. The feasibility of a solid-liquid ion exchange reaction depends on the affinity between the solid ion exchanger and the ion to be removed from the solution, on the nature of such solution, on the capacity of the ion exchanger for the desired ion, and on the selectivity of the ion exchanger for the desired ion in the given system. Thus, while ion exchange resins are extensively used to recover uranium from sulfate media, attempts to develop industrial scale processes for the recovery of uranium from WPA failed because the available resins did not have enough capacity and selectivity for uranium. Thus for example, R. Derry in "The Recovery of Uranium from Phosphatic Sources in Relation to EEC", Report No. EUR-7324, pp. 24-27, EEC, Brussels (1981), describes some unsuccessful attempts for the recovery of uranium from wet process phosphoric acid by ion exchange and the state of the art is summed up on page 27 by the statement that "the lack of selectivity, however, still remains a problem area with ion exchange resins systems".
Another attempt at the extraction of uranium from a 30% P.sub.2 O.sub.5 WPA by means of ion exchange is described by Irvin R. Higgins in a review paper entitled "Hydro Metallurgical Recovery of Metal Values by the Use of Ion Exchange". The American Institute of Chemical Engineers, Symposium Series, Vol. 78, No. 216 (1982) at page 147. According to that disclosure a weak base resin was used for the purpose and stripping was effected with aqueous Na.sub.2 CO.sub.3 but, as the author states, the extraction coefficient was so low that prodigious amounts of Na.sub.2 CO.sub.3 strip agent were required.
In U.S. Pat. No. 4,002,564 (Carbonet et al.) there is described a group of ion exchange resins comprising each a cross-linked vinyl-aromatic polymer carrying recurring active aminophosphonic units of the formula --CH.sub.2 NHRPO(OH).sub.2 wherein R is a lower alkylene radical. It is stated in the specification (column 1 lines 44-46) that such ion exchange resins are capable of selectively removing metallic ions from aqueous solutions and in column 3 lines 42-46 it is further stated that the removal of metallic ions from aqueous solution with the aid of these cation-exchange resin is "conventional in so far as operating conditions are concerned such as pH, temperature, concentration, and the like".
A related group of cation-exchange resin is described in French patent specification No. 2,489,711 to Minemet Recherche. The resins there described are characterised by active hydroxy phosphonic groups of the formula ##STR1## where R is propyl, isopropyl, ethyl, methyl or hydrogen and A is optionally substituted ethylene or methylene. On page 4 of the French specification it is mentioned that such a cation-exchange resin may be used for the recovery of uranium from phosphoric acid and that for this purpose it is required to contact the uranium bearing phosphoric acid with the resin, if desired after preliminary reduction, and then to elute the uranium in an oxidizing medium by means of an alkali or ammonium carbonate.
A particular resin out of the group of those disclosed in the said U.S. Pat. No. 4,002,564 has functional groups of the formula --CH.sub.2 NH--CH.sub.2 --PO.sub.3.sup.-2 which are attached to a macroporous polystyrene matrix and is known under the trade name Duolite ES 467 (Dia-Prosim). In a pamphlet dated August 1981 the manufacturers state with respect to this ion exchange resin that "uranium can be recovered from concentrated (30%) phosphoric acid solutions" but there is no teaching as to how this may be achieved.
Because of the close relationship between the cation exchange resins according to U.S. Pat. No. 4,002,564, including Duolite ES 467, and those of French Pat. No. 2,489,711 and seeing that neither the said U.S. patent nor the said pamphlet include any specific instructions as to how to recover uranium from WPA with the subject cation exchanger, attempts have been made by the present inventors to proceed in accordance with the teachings in French patent specification No. 2,489,711. Accordingly, crude WPA was subjected to reduction with iron powder so as to reduce the uranium from the hexavalent to the tetravalent state, the so-reduced WPA was then contacted with the resin and the loaded resin was eluted with aqueous sodium or ammonium carbonate. The results were very unsatisfactory and among the problems that were encountered there may be mentioned the fact that the organic matter in the crude acid fouled the resin and was in part carried over into the eluate thereby interfering adversely with the precipitation of the uranium product, the so-called "yellow-cake". Moreover, the stripping coefficient of uranium with aqueous sodium or ammonium carbonate was low, which meant long tails and large volumes of eluate or low uranium concentration. Further difficulty was due to the fact that the acidic cation exchanger reacted with the carbonate eluting agent resulting in the formation of gaseous carbon dioxide which interfered adversely with the elution process.